The present invention relates to a grinding machine which is suitable for machining a workpiece to a very high degree of accuracy.
In order to perform machining such as grinding of a plane face of a workpiece to a very high degree of accuracy, it is essential that the workpiece can be precisely and reproducibly set in a predetermined position and orientation with respect to the cutting tool. This precision must be maintained as a number of workpieces are successively machined and removed from the machine. In the prior art, it has been necessary to use substantially expensive and complex measuring means in order to achieve such a precision of workpiece positioning accuracy, and in addition the time consumed in setting each workpiece into the requisite position and orientation results in a substantial increase in the time required for machining to be carried out.
In order to perform machining with a very high level of accuracy, the following four conditions must basically be met:
1. Improvements of the method of machining
2. Enhancement of the accuracy of guiding the various elements involved in machining.
3. Improvement of the rigidity or "stiffness" of the feed system which presses the cutting tool against the workpiece to perform machining, and reduction of deviations in the position of the cutting tool caused by that feed system.
4. Assurance of position stability of the workpiece with respect to vibration and heat.
Each of the above requirements will now be discussed. As regards the first requirement, for improvement of the machining method, it has been general practice in the prior art for the tool shaft of a grinding machine (i.e. a grinding machine in which that shaft is vertically disposed) to be supported by a ball-roller bearing. Using such a bearing results in a low degree positioning accuracy for the tool shaft, and hence for the cutting tool, in the vertical direction. This results in a lowering of the accuracy of machining. In addition, it is difficult to precisely control the machining force applied to the workpiece when such bearings are utilized. However with the present invention, a configuration is adopted whereby tool mounting head in which the tool shaft rotates is supported by a hydrostatic bearing provided on the elevating feed screw which raises or lowers the tool mounting head. A load compensation system is coupled to the tool mounting head and controlled by pressured difference developed within this hydrostatic bearing to operate to effectively support the weight of the tool mounting head. In this way, positioning of the cutting tool in the vertical direction is determined very precisely by rotation of the elevating feed screw, since no weight actually supported by the threads (and hence by a lubricating oil film thereon) of the elevating feed screw.
Use of this system enables the amount of tool shaft deviation to be held to less than 0.1 micron, even if the workpiece feed rate is of the order of 1 m/minute, so that the present invention enables a substantially higher degree of productivity to be attained for ultra-high precision machining than has been possible with prior art types of grinding machines.
With regard to the second condition, concerning guidance precision of the tool shaft, the present invention utilizes a static pressure guide system in which hydrostatic bearings are disposed along the workpiece slide. The workpiece slide traverses a cross slide guide surface over with a rotary table being rotatably mounted on the workpiece slide. In prior art grinding machines, the tool mounting head is generally positioned for vertical movemement by what are sometimes called hydrodynamic guide surfaces. An opposing guided surface on the tool mounting head slides relative to a guide surface, through the intermediary of a lubricating film of oil or grease, to provide smooth motion of the head in the vertical direction. However the lubricating film formed between each guide surface and the opposing guided surface does not in itself have the capability for restraining the tool mounting head, and hence the axis of rotation of the tool shaft, from tilting out of the vertical. Thus, when a tilting moment is applied to the tool mounting head, as a result of external forces being applied due to machining operations or due to a displacement of the center of gravity of the tool mounting head, then there will be insufficient resistance to this tilting moment. The tool mounting head will therefore become tilted by an amount which is determined by the size of the gap between the guide surace and the opposing guide surface. This makes the orientation of the cutting tool with respect to the workpiece to be unstable, thereby diminishing the accuracy of machining.
With the present invention, however, a compound guide arrangement is employed. In this case, at least one guide surface provided on the supporting columns serves as a reference guide surface, with a thin film of high pressure lubricant being forced between that reference guide surface and the opposing guided surface. A sufficient amount of frictional resistance is developed between this reference guide surface and the opposing guided surface to act to oppose forces applied to the tool shaft by machining operations, while at the same time the thin film of lubricant enables smooth movement of the tool mounting head in the vertical direction to be achieved. A plurality of other guide surfaces are also provided on the supporting columns, opposing adjacent guided surfaces formed on the tool mounting head, and these function as hydrostatic guide means, with pressurized fluid at constant pressure being forced between these guide surfaces and guided surfaces. As result of this static pressure fluid flow, forces are developed to counteract any tilting of the tool mounting head, and hence the tool shaft, so that the tool shaft is maintained in a precisely vertical orientation during raising and lowering of the tool mounting head, irrespective of forces applied due to machining. At the same time, the hydrostatic fluid flow serves to keep the guide surfaces and guided surfaces from coming into contact, so that there is negligible frictional resistance developed between these surfaces.
With regard to the third condition, i.e. improvement of the rigidity of the feed system, the static pressure bearing provided on the elevating feed screw (which is rotated to raise or lower the tool mounting head) and the load compensating system controlled thereby, referred to above, serve to provide a substantial improvement in this respect. For example in a prior art system in which such a static pressure bearing is not incorporated, a certain amount of "looseness" will be present in the feed mechanism for elevating the tool mounting head, due to the need to provide a layer of lubricant between the feed screw threads and the corresponding threads in which they engage, and due to the fact that the weight of the tool mounting head assembly is essentially supported by these threads and hence is applied to the lubricant layer on the threads. However with the present invention, since as described above the entire weight of the tool mounting head assembly is supported by an actuating rod controlled by a load compensation system (in the preferred embodiment, a hydraulic feedback loop), there is negligible load applied to the threads of the feed screw, so that an extremely "stiff" and rigid feed mechanism is attained.
With regard to the fourth condition, i.e. stability of machining accuracy with respect to vibration and heat, electric motors are employed for driving the mechanism which develop an extremely low amount of vibration. These motors are coupled through elastic coupling means to the driven members, in such a way that a negligible amount of motor shaft vibration is transmitted directly to the driven members. With regard to heat problems, the temperature of operation of the grinding machine is essentially determined by the temperature of the lubricating oil. For this reason, a special oil filter system is employed which also performs cooling of the oil, in such a way as to hold the temprature of the oil exiting from the filter system at a predetermined temperature, with only a small amount of fluctuation.